US 6906905 B1
A three-dimensional micro electro-mechanical (MEMS) variable capacitor is described wherein movable comb electrodes of opposing polarity are fabricated simultaneously on the same substrate are independently actuated. These electrodes are formed in an interdigitated fashion to maximize the capacitance of the device. The electrodes are jointly or individually actuated. A separate actuation electrode and a ground plane electrode actuate the movable electrodes. The voltage potential between the two electrodes provides a primary mode of operation of the device. The variation of the sidewall overlap area between the interdigitated fingers provides the expected capacitance tuning of the device. The interdigitated electrodes can also be attached on both ends to form fixed-fixed beams. The stiffness of the electrodes is reduced by utilizing thin support structures at the ends of the electrodes. The three dimensional aspect of the device avails large surface area. Large capacitance variation and tuning ranges are obtained by independent actuation of the electrode fingers. A plurality of modes of operation of the device provides wide flexibility and greater performance advantage for the device. Upon fabrication of the device, a separate substrate with etched dielectric is used to encapsulated the device. The MEMS device is then completely encapsulated, requiring no additional packaging of the device. Further, since alignment and bonding can be done on a wafer scale, an improved device yield is obtained at a lower cost.
1. A micro electro-mechanical system (MEMS) variable capacitor comprising:
a fixed electrode formed on a substrate; and
two movable beams facing each other, each of said movable beams being respectively anchored to said substrate at at least one end thereof, said movable beams being formed by co-planar metal lines interconnected by a plurality of conductive vias, said conductive vias and said metal lines being embedded in insulating material, said movable beams further comprising a bottom electrode facing said fixed electrode, formed on the bottom surface of said insulating material, wherein the capacitance varies as a function of the sidewall overlap area of said two movable beams rotating with respect to one another.
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14. A micro electro-mechanical system (MEMS) variable capacitor comprising:
a plurality of fixed electrodes formed on a substrate parallel to each other; and
movable beams facing each other and facing one of said fixed electrodes, each of said movable beams being respectively anchored to said substrate at at least one end thereof, said movable beams being formed by co-planar metal lines interconnected by a plurality of conductive vias, said conductive vias and said metal lines being embedded in insulating material, said movable beams further comprising a movable electrode facing said fixed electrode, formed on the bottom surface of said insulating material, wherein the capacitance varies as a function of the total sidewall overlap area of said movable beams.
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This application is related to U.S. Patent application Ser. No. 10/710,286, Elastomeric Micro Electromechanical Varactor, filed concurrently, and which is incorporated herein by reference.
This invention is generally related to micro-electromechanical system (MEMS) devices, and more specifically, to a variable capacitor that uses three-dimensional comb-drive electrodes which can be integrated into current state of the art semiconductor fabrication processes.
Variable capacitors or varactors play a fundamental role in high-frequency and radio-frequency (RF) circuits. In the last few years, MEMS variable capacitors have drawn considerable interest due to their superior electrical characteristics.
While variable capacitors using MEMS technology can be readily implemented in standard semiconductor devices for applications in aerospace, consumer electronics and communications systems, researchers have attempted to improve the tuning range of MEMS variable capacitors since the maximum capacitance tuning range achieved by parallel plate electrodes is limited. This is due to the non-linear electrostatic forces involved during actuation. Parallel plate electrodes exhibit a typical “pull-down behavior” at one-third the gap distance, leading to a maximum tuning capacitance of 1.5. Most previous approaches have resulted in an increased processing complexity and/or a large number of moving parts, leading to a drastic reduction in reliability. Additionally, packaging MEMS devices and integrating them into CMOS integrated circuits pose great challenges.
A. Dec et al., in an article entitled “RF micro-machined varactors with wide tuning range”, published in the IEEE RF IC Symposium Digest, pp. 309-312, June 1998, describe the construction of a MEMS variable capacitor by actuating a movable electrode using two parallel electrodes above and below the movable electrode. The total capacitance tuning range is significantly enhanced as a result of the individual capacitance between the movable-top and movable-bottom being in series. The maximum tuning range achievable using this approach is a ratio of 2:1. A. Dec et al. report achieving a tuning range as high as 1.9:1. Even though the tuning range significantly improves when using this approach, the process complexity increases correspondingly to a level that significantly reduces their utility for industrial applications.
U.S. Pat. No. 6,661,069 describes a method of fabricating a micro electro mechanical varactor using comb-drive electrodes as actuators. This approach is intended to increase the tuning range, but its construction, as described, involves fabricating the device on two separate substrates. The primary mode of actuation resides between the fin structures within the device. Further, the device is a three-port varactor and does not offer multiple actuating modes for enhancing the tuning range of the device.
In view of the foregoing considerations, there is a distinct need in industry for variable capacitors which construction differs considerable from the parallel plate devices and which method of fabrication differs from the conventional methods previously discussed. In particular, what is required are movable comb-drive electrodes for capacitance sensing and separate actuation electrodes for actuation of the movable comb drive electrodes. Preferably, the capacitance should vary by actuating one or more of the electrode fingers, thereby varying the overlap area between the comb electrodes. The capacitance tuning range of such device requires to be greatly enhanced by taking full advantage of multiple modes of actuation if possible in such devices. Since multi-port capacitors are required (i.e., at least two ports for DC bias and two ports for the RF signals), the signal capacitance should not require decoupling as is the case in conventional three-port varactors. The device should further be fabricated using standard semiconductor fabrication techniques and allow for an easy integrated into semiconductor circuits.
Accordingly, it is an object of the invention to provide a MEMS variable capacitor that utilized multi-fingered interdigitated three dimensional comb drive electrode for sensing, while the control or actuation electrodes drive the motion of the movable comb drive electrode beams either individually or all in unison, leading to changes in capacitance. It is another object to provide a MEMS varactor wherein the switch contacts are separated by a dielectric to provide electrical insulation between the ground electrode and the actuation electrode.
It is further an object to provide a MEMS variable capacitor with comb-drive electrode sensing for obtaining large capacitance ratio or tuning range.
It is yet another object to configure a plurality of MEMS variable capacitors in a variety of three-dimensional configurations.
It is still another object to provide a MEMS varactor having controlled stress gradient in the comb-drive electrode fingers leading to large change in overlap area.
It is still another object to provide a MEMS variable capacitor wherein the number and type of support structures to the movable comb drive fingers vary to lower the drive voltages.
It is still a further object to provide a method of fabricating a MEMS variable capacitor using manufacturing techniques that are compatible with applicable to CMOS semiconductor devices, which allows fabricating and packaging the MEMS device simultaneously and reduces the number of fabrication steps to a minimum while reducing the cost of processing.
MEMS based variable capacitors provide many advantages over conventional solid-state varactors. These devices operate at higher quality factors leading to low loss during operation. Two types of MEMS varactors are described herein: parallel plate and comb-drive varactors.
Most widely investigated MEMS varactors are parallel plate capacitors with a movable electrode and a fixed electrode. The major disadvantage when using these MEMS devices is the limited tuning range of operation obtained upon actuation of these devices. The inherent electro-mechanical aspects involved restrict the tuning range and lead to snap down of the movable electrode. This is often referred to as the “pull-down instability effect”. Electrostatic forces acting on the movable electrode are non-linear in nature, causing this effect. On the other hand, in the comb-drive electrodes, the electrostatic forces acting on the movable electrode are linear (i.e., directly proportional to the distance) which greatly enhances the tuning range. However, comb-drive electrodes are difficult to process and the change in capacitance obtained is very small (due to less area available).
In one aspect of the invention, the MEMS variable capacitor described includes both of the approaches, i.e., parallel plate and comb-drive capacitors that were thus far considered. A greater area is made available during tuning by fabricating a three-dimensional multi-layered electrodes in a comb-drive configuration. The non-linear electrostatic forces from the parallel plate approach are utilized to provide independent or simultaneous actuation to the comb drive electrodes. The movable and fixed electrodes are processed sequentially on a single wafer. The intrinsic stress gradient in the film stack, metal layer and the metal interconnections is used to form curved beams of controlled topography. Devices having separate DC ports for actuation and RF ports for sensing are formed using this configuration, the RF (signal) electrodes being formed by the comb drive electrodes, and the actuator electrodes formed underneath the movable electrodes, provide actuation to the movable beams. The ground plane electrode is electrically isolated from the sensing comb-drive electrodes by lack of inter-level vias.
After completion of the processing and release of the MEMS variable capacitor, the device is advantageously packaged and encapsulated in dielectric by utilizing a second carrier wafer with trenches and precision aligning to completely cover the released MEMS structure. The height of the trench on the carrier wafer is determined by the maximum tip deflection of the movable comb drive electrode. Finally the device is encapsulated with polymeric material in order to provide controllable environment for the MEMS device during operation.
In a second aspect of the invention, the actuation electrodes underneath the movable comb-drive electrodes are combined to provide a single actuation for all the electrodes having the same polarity. The electrodes with opposite polarity are separately actuated. The inventive variable capacitor operates under four modes of actuation, thereby leading to a change in capacitance in each of the four modes.
In a third aspect of the invention, the actuation electrodes underneath the movable comb-drive electrodes are individually actuated for electrodes having opposite polarity and, hence, they provide numerous states or modes of operation. For each of the modes, the capacitance of the device changes when compared to equivalent prior state devices. The capacitance tuning of the device is greatly enhanced by gradually stepping up the actuation for the device.
These and other objects, aspects and advantages of the invention will be better understood from the detailed preferred embodiment of the invention when taken in conjunction with the accompanying drawings.
The present invention will now be described more fully, hereinafter with reference to the drawings, in which preferred embodiments are shown.
In the preferred embodiment, the metal connections and electrodes are, preferably made of copper, with a suitable liner and barrier material, such as Ta, TaN, Ti, TiN, W, and the like. Each metal conductor in electrode 20 is approximately 5000 Å-8000 Å thick. Conductor 85 is illustrative of an actuation electrode, wherein the gap separating electrodes 85 and 86 determines the actuation voltage of the device.
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While the invention has been described in conjunction with a preferred embodiment, it is to be understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the aforementioned description. Accordingly, it is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims. All matters set forth herein or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.